1. Field of the Invention
The present invention relates to a vehicle suspension system, and more particularly, to a vehicle suspension system for a vehicle such as an automobile which is improved with respect to the roll control in the turning of the vehicle over a wide speed range from low vehicle speed to high vehicle speed.
2. Description of the Prior Art
In a vehicle such as an automobile it is generally desirable that the US-OS performance is set on the oversteer (OS) side from the view point of improving the responsiveness to steering in the turning of the vehicle, while it is desirable that the US-OS performance is set on the understeer (US) side from the view point of improving the stability of steering in the turning of the vehicle. In view of this, in, for example, Japanese Patent Laid-open Publication 62-198511 it is described to operate the suspension control means with the US-OS performance being set on the oversteer side of neutral in the beginning of a turning of the vehicle and on the understeer side near the end of the turning of the vehicle.
According to such a suspension control means a quick steering is available in the beginning of a turning of the vehicle due to an oversteer or neutral performance of the suspension control, and a stable steering operation is available near the end of the turning of the vehicle due to an understeer performance of the suspension control.
However, since in such a suspension control means the US-OS performance is set on the oversteer side or neutral in the beginning of a turning regardless of the vehicle speed, the stability of steering lowers when the vehicle turns at high vehicle speed.
Further, it is already known as described in, for example, Japanese Patent Laid-open Publication 63-17185 to construct a four wheel steering means to be responsive to vehicle speed so that the front and rear wheels are steered with a variable front to rear wheel steering angle ratio which is varied from a substantial value in the opposite phase to a substantial value in the same phase along with increase of vehicle speed so that both a small radius turning a low vehicle speed and a high steering stability at medium to high vehicle speed are available. Such a four wheel steering means can provide a very good turning performance as compared with a steering control means in which front wheels only are steered.
However, since such a four wheel steering means increases the steering angle in the rear wheels in the direction of the same phase along with increase of vehicle speed, a rolling of the vehicle body is apt to occur in an early stage of turning. Further, since in such a four wheel control means the front to rear wheel steering angle ratio is set to a high value in the same phase at high vehicle speed so that the US-OS performance is in an understeer performance, the responsive to the steering in a transitional state in a turning at high vehicle speed is low.
In view of the above-mentioned problem it is a first object of the present invention to provide a vehicle suspension system which is improved in ensuring the stability of steering in the turning at high vehicle speed without sacrificing the quick steering performance in the turning at low vehicle speed.
Further, in view of the above-mentioned problem in a vehicle having such a conventional four wheel steering means ,it is a second object of the present invention to provide a vehicle suspension system which is improved in the steering performance in a transitional state in a turning, particularly, the steering performance in a transitional state in a high speed turning while ensuring the stability of steering in high speed running.